The present invention relates to a method of eliminating warp distortions occurring in the course of forming thin-piece samples for use in transmission electron microscopes (TEM), etc., using a focused ion beam (FIB).
Section TEM samples are required to be formed so that the desired point of observation is at a thickness through which an electron beam can be transmitted. Conventionally, it has been well known that an ion beam may be used to form a test/examination sample into a desired shape. Forming a sample for use in a transmission electron microscope (TEM) is performed by cutting the sample, using a focused ion beam, to a proper thickness so that electrons can transmit through the sample. This sample forming method is disclosed in the reference entitled, xe2x80x9cSection TEM Sample Forming Method Using a Focused Ion Beamxe2x80x9d, 37th Applied Physical Academy, March 1990. Also, Japanese Patent Application No. 192641/1990 (JP-A-4-76437), filed by the present applicant, is also related to this technology. When a sample is machine-polished down to approximately several 10 xcexcm, it is further cut at opposite surfaces of an observational point by ion beam etching so as to leave a thin wall of less than 1 xcexcm, which makes it more convenient to confirm and observe a forming position, forming shape, section, etc. The prior application discloses an invention where a scanning electron microscope irradiating an electron beam is arranged so as to form a sample while monitoring a forming portion of the sample. Furthermore, disclosure is also made of the technology where the sample surface is locally formed with a film so as to prevent any damage due to the ion beam, and the inclination angle of a forming surface resulting from a convergent angle of the ion beam is compensated for by using an inclination setting in a table holding the sample in order to produce evenness in the sample thickness.
A summary of the basic technology of a focused ion beam forming apparatus for the present invention will be explained referring to an embodiment of the prior application shown in FIG. 3.
When a sample 4 to be formed is placed on a stage 5, the sample chamber is placed in a vacuum state by a vacuum apparatus (not shown). The sample stage 5 is set to a desired positional angle by a drive mechanism (not shown). The drive mechanism, in general, is capable of: (1) displacement in the X, Y and Z directions, (2) ion beam axis rotation, and (3) adjustment in angle relative to an ion beam axis. When a forming region is determined, a region of the sample having an end portion of a thin wall is exposed to a chemical vapor deposition (CVD) gas from the gas gun 9 in order to prevent damage to the portion by the ion beam, and a metal protection film is formed. Next, the forming region is irradiated by an ion beam and cut by sputtering. In this case, the relative displacement of the ion beam 2 and the sample 4 is made by scanning the ion beam with the deflection means of the electrostatic optical system 3, without using a drive device, because the forming requires extreme precision on the order of microns. Initially, the ion beam current is adjusted to increase the sputter rate in order to shorten the process time, thereby performing rough forming. Finally, the ion current is decreased in the area of the sample forming region, thereby performing precision forming. The feature of this apparatus lies in a structure where an electron beam can be irradiated to a sample for sample surface observation in a direction different than that of the ion beam. Because of this arrangement, an electron beam 7 may be solely used for scanning a sample that would have been damaged by an ion beam. The secondary charged particles (electrons) 11 are then detected by the secondary charged particle detector 10. Thus, observations may be made at the same time the samples are formed without taking the samples out of the apparatus. Also, because scanning electron microscope (SEM) images and scanning ion microscope (SIM) images are different due to the different kinds of secondary charged particles 11 emitted from the sample, images having different resolutions may be obtained. Accordingly, both images can be compared side-by-side on a display 13.
FIGS. 2A to 2D refer to the forming of a sample for use in a transmission electron microscope (TEM). A holding piece (not shown) is fixed with a sample block 4 that is mechanically cut out and is placed on a sample table (stage) 5 of the focused ion beam apparatus through a holder (not shown), and an ion beam 2 is irradiated to form the sample (see also FIG. 3). The specific procedure is as follows. In the first step, as shown in FIG. 2A, a sample 4 is a sectionally convex-formed block formed by mechanical cutting, and the sample is placed on the sample stage 5 (see FIG. 3). Next, a forming frame of the sample is determined, and, in order to prevent damage by the irradiating ion beam to the end portion being formed into a thin wall, a CVD gas (e.g., phenanthrene C14H10) is applied to that portion, thereby forming a protective coating layer 41 (as shown in FIG. 2B). In the next step, an ion beam 2 is irradiated and the sample block is cut at opposite surfaces by sputtering. Thus, a thin wall 42 of a sample becomes formed as shown in FIG. 2C. This sample for use in a transmission electron microscope has no differences in the shape of the opposite surfaces, and the sample is required to be of a thickness where an incoming electron beam 7 to the thin wall from a perpendicular direction can transmit through the thin wall (being 0.5 xcexcm or less), as shown in FIG. 2D.
When the reduction of thickness of the sample is proceeded by ion beam forming, there is a chance that the thin-walled portion of the sample will become distorted and warped, as if a celluloid board is bent by pressing at opposite ends. If the sample becomes distorted or warped, and even if an ion beam is linearly scanned, it becomes impossible to perform planar forming with an even thickness because of the distortion. FIG. 1A is a top view of a sample having a thin wall where warp distortion occurred. Conventionally, when distortion occurs, the sample block had to be replaced and the forming operation had to be performed again from the beginning. However, where the samples are composed of the same materials and the structures are the same, if distortion occurs once during forming, then chances are that distortions will be unavoidable in forming samples composed of similar materials and having similar structures.
Another disadvantage is that if the sample forming is suspended at a stage of thickness before the deformation occurs, electron transmissibility may be too low, or non-coincident portions may be left in the opposite surfaces, which becomes unsatisfactory for use as a TEM sample. Referring to FIG. 1C, if pillars 43 are left in the midway of an observational region and the thin wall 42 width is narrow, the phenomenon of warp distortion will not occur. However, in such a case, the observational region is limited, but more importantly, this type of forming requires much time and labor, and therefore this type of forming is not preferred.
The present invention is aimed at providing a forming method capable of easily and more efficiently preparing a thin-piece sample for use in a transmission electron microscope by eliminating warp distortion of a sample during the formation process of thin-piece sample.
The present invention is directed to a method of removing warp distortions of a thin-piece sample for use in an electron microscope during the formation of the sample. The sample is cut to form a fissure in a stress concentration portion of the sample using an ion beam until the warp distortions disappear. The stress concentration often occurs at a device region having material anisotropy in the sample, and the distortion is removed by forming a fissure in the depth direction of the sample.